(i) Field of the Invention
The present invention relates to a ceramic jig such as a guide roller, a rolling roll or a guide tube which can be suitably used as a jig for the hot rolling of steel or alloys.
(ii) Description of the Related Art
Heretofore, jigs such as a guide roller and a rolling roll which can be used in a hot rolling line for manufacturing bars, wires, tubes or the like have been mainly made of metals.
However, these metallic hot rolling jigs are usually easily worn, and so these jigs have a short life. The jigs made of WC super hard alloys and the like which are excellent in wear resistance have been developed, but even such jigs are poor in heat resistance. Therefore, there is a problem that when the jig comes in contact with a material to be rolled, chap and baking tend to occur. In addition, these metallic jigs are heavy, which makes the attachment or the exchange operation of such jigs difficult.
In recent years, as jigs which can be substituted for such metallic jigs for hot rolling, there have been intensively investigated ceramic jigs for hot rolling which can utilize excellent characteristics of ceramics such as wear resistance, heat resistance and light weight. As a constitutional material of such a ceramic jig for hot rolling, a sintered silicon nitride which is known a high strength ceramics is mainly used, but silicon nitride is a substance which can scarcely be sintered. Thus, in the manufacture of the jig from the silicon nitride material, an oxide such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 or MgO is usually added as a sintering aid in order to accelerate the sintering via a glass phase formed among silicon nitride particles.
However, when the jig for hot rolling comprising a conventional silicon nitride sintered body which has been manufactured as described above is used in a hot rolling line of steel or alloys, the remaining glass phase among the Si.sub.3 N.sub.4 grains reacts with iron in the material to be rolled and cooling water, so that the jig is corroded and thus it is noticeably worn during its use inconveniently.